The present invention relates to an optical module, a transmitter, a receiver, an optical switch, an optical communication unit, an add-and-drop multiplexing unit, and a method of manufacturing the optical module.
In recent years, measures have been considered in various circles for providing silica waveguides, as well as such optical integrated circuits as optical splatters, wavelength division multiplexers/demultiplexers, optical switches, and the like, which use a silica waveguide to enhance the functions of optical parts for communications, as well as to reduce the size and cost of those parts. Measures have also been considered for obtaining high performance optical modules to be realized by mounting a semiconductor optical device, such as laser diodes and photo-diodes, on a substrate with optical integrated circuits. Those optical modules, when used for a wavelength division multiplexing (WDM) transmission unit and an optical add-and-drop multiplexing (ADM) unit, can enhance the performance of the object communication system, as well as cut down the size and cost of the same significantly. Optical modules, in each of which a semiconductor optical element is mounted in a silica waveguide, are described collectively in the Technical Digest of Third Optoelectronics and Communications Conference, Makuhari, Japan, p. 370-371 (1988). In this document, a WDM light source module is realized with semiconductor amplifiers mounted on a substrate with a waveguide array having diffraction gratings on a substrate. And, a wavelength converter module is realized with a semiconductor optical amplifier mounted on a substrate with a 3 dB coupler circuit. Furthermore, a fast wavelength filter is realized with semiconductor optical amplifiers mounted between two arrayed waveguide grating (AWG) wavelength multiplexers/demultiplexers.
On the other hand, not only silica waveguides but also polymer waveguides have been under examination. A polymer waveguide is fabricated by coating a silicon (Si) substrate with varnish obtained by dissolving a polymer in a solution.
Consequently, when compared with the silica waveguide, the mass productivity is higher and the cost is lower. The polymer waveguide also has a large thermo-optical coefficient. If such a polymer waveguide is used, therefore, it is possible to compose an optical integrated circuit, such as a wavelength tunable filter and a digital optical switch, with enhanced functions which have never been realized in a silica waveguide. A wavelength tunable wavelength division multiplexer is described in, for example, IECE Transactions on Electronics, Vol. 7, p.1020-1026 (1026) and a digital optical switch is described in the Technical Digest of the Third Optoelectronics and Communications Conference, Makuhari, Japan, p.66-67 (1998). Just like the silica waveguide, mounting a semiconductor optical element in a polymer waveguide or in an optical integrated circuit which uses such a polymer waveguide will result in a high performance optical module.
In spite of the above-mentioned favorable characteristics of the polymer waveguide, there are still some problems which must be solved. For example, in order to make sure that a high optical coupling efficiency is obtained between a waveguide type semiconductor optical element mounted on a substrate and a polymer waveguide or a polymer optical integrated circuit fabricated on the same substrate, the height of the light axis of the polymer waveguide must be aligned with the height of the light axis of the semiconductor optical element. At this time, the refractive index difference between the core layer and the cladding layer of the polymer waveguide is usually set to 0.3 to 1% and the thickness of the core layer is set to 5 to 8 xcexcm considering the loss of coupling with an optical fiber, the fabrication tolerance of the optical circuit, and the size of the optical circuit. If a polymer waveguide is formed on an Si substrate, a lower cladding layer must be formed at a thickness of 10 xcexcm or more so as to suppress an increase of the propagation loss and the polarization dependent loss (PDL), which are caused by the Si substrate. Consequently, the height of the core layer in the center portion from the surface of the substrate becomes 13 xcexcm or more. On the contrary, the height of the core layer of the semiconductor optical element in the center portion from the surface of the substrate is at most 5 to 12 xcexcm when the optical element is flip-chip bonded on the substrate. The difference in height between those items becomes at least 1 xcexcm, so that the optical coupling loss between the semiconductor element and the polymer waveguide becomes very large. In order to align both of those items in height, two methods have been proposed. According to the first method, a projection referred to as a terrace is formed at a portion of the Si substrate, where a semiconductor element is mounted. The method already has been applied to silica waveguides. In this case, however, the manufacturing will become difficult, since both polymer and Si must be polished and flattened simultaneously when this method is applied to forma polymer waveguide. The second method applicable to such a height alignment of the polymer waveguide is described in IEICE technical report, EMD98-55 (1988). According to this method, part of the lower cladding layer on which a semiconductor element is to be mounted is left as is, and then the element is mounted on the left-over lower cladding layer (referred to as a pedestal). This method, however, also creates problems in that the temperature characteristics of the laser diode are degraded, since the laser diode comes to be mounted on a polymer with a low thermal conductivity, and a metallic layer must be formed in the middle of the lower cladding layer so as to stop the etching at a predetermined height.
Under such circumstances, it is an object of the present invention to provide an optical module which solves the foregoing problems and may be produced at lower prices than conventional ones.
The first object of the present invention is therefore to propose a method and a structure for matching the height of the polymer waveguide to the height of the core layer of a semiconductor element with less degradation of the characteristics and with fewer fabrication processes in an optical module provided with a waveguide type semiconductor element mounted on its substrate provided with a polymer waveguide or an optical integrated circuit composed of such a polymer waveguide, thereby providing an optical module of higher performance and lower cost than conventional ones. The second object of the present invention is to provide an optical communication unit which uses an optical module so as to enhance the functions and reduce the cost to an extent greater than conventional ones.
The present invention is characterized by an optical module which is composed as follows: At first, a polymer waveguide is formed at a portion of a silicon substrate coated with an oxide silicon film thereon, so that
the relational conditions of d greater than xcex/2xcfx80xc2x7(ncore2xe2x88x92nSiO22)xc2xd) are satisfied when it is assumed that the thickness of the oxide silicon film is d, the refractive index of the oxide silicon film is nSiO2, the refractive index of the core layer is ncore, and the wavelength of a light transmitted through the polymer waveguide is xcex. Then, a semiconductor optical element is provided at another portion of the silicon substrate, and an end face of the waveguide is coupled with an end face of the semiconductor optical element optically within a predetermined error range, and the thickness of the substrate where the waveguide is provided is practically the same as the thickness of the substrate where the semiconductor optical element is provided in a cross sectional view of the-substrate.
The first object of the present invention described above is achieved as follows: At first, a polymer waveguide provided with a lower cladding layer, a core layer, and an upper cladding layer, or an optical integrated circuit which uses the waveguide, is provided on a silicon substrate coated with an oxide silicon film. Then, part of the polymer waveguide is removed from the substrate, so that a semiconductor optical device is bonded on an electrode formed on the oxide silicon film on the substrate from which the polymer waveguide already has been removed. Here, the lower cladding layer is thinned more than in the conventional device until the height of the polymer waveguide is aligned with the height of the core layer of the optical semiconductor in the center. The core layer thus becomes 5 to 12 xcexcm in height in the center portion. Accordingly, the lower cladding layer is set to 3 to 9 xcexcm and the core layer is set to 5 to 8 xcexcm in thickness. Then, both d and ncore values are set so as to satisfy the following relational conditions among the thickness of the oxide film (d), the refractive index (nSiO2), the refractive index of the core layer (ncore), and the wavelength of the light transmitted in the waveguide (xcex). This is to prevent the leakage of the light to the Si substrate.
D greater than xcex(2xcfx80xc2x7(ncore2xe2x88x92nSiO22)xc2xd)
Since the thickness of an oxide film to be formed easily with a thermal oxidization method is not more than 1.5 xcexcm, the ncore value is set to 1.47 or more. In addition, the ncore should be set to 1.55 or less so as to reduce the reflection of the light at the boundary between an end face of the waveguide and an optical fiber. Accordingly, the d value should be set to 0.4 xcexcm or over. Taking into consideration the loss of the optical fiber at the connections, as well as the fabrication tolerance of the optical circuit, the refractive index of the core layer is set 0.3 to 1% larger than that of the upper/lower cladding layer. Fluorinated polyimide can be used as a polymer material for the lower cladding layer, the core layer, and the upper cladding layer. A semiconductor laser diode or a waveguide type photo-diode can be used as the semiconductor elements and a branching circuit, an arrayed waveguide grating wavelength division multiplexer/demultiplexer, an optical switch, or the like can be used as the optical integrated circuit and a plurality of laser elements or a laser array, each having a different oscillation wavelength, can be used to compose a semiconductor element and an optical branching circuit can be used as an optical integrated circuit, thereby composing a WDM transmitter module. And, a plurality of waveguide photo-diodes or a photo-diode array can be used to compose a semiconductor element and a waveguide array grating wavelength division multiplexer/demultiplexer can be used as an optical integrated circuit, thereby composing a WDM receiver module. In the same way, a waveguide photo-diode can be used as a semiconductor element and an optical switch can be used as an optical integrated circuit, thereby composing an optical switch with monitoring functions.
The second object of the present invention described above can be achieved with an optical transmission unit, as well as an optical add-and-drop unit using the optical modules described above.
Another layer can also be formed between the substrate and a polymer waveguide, which is to be used as the waveguide described above. The layer should preferably be a thin film layer.